Since the traffic volume of trunk communication systems has been drastically increasing due to widespread use of the Internet, there are great hopes that an ultra high speed optical communication system in excess of 40 Gbps will be practically implemented.
In recent years, as a technology to realize an ultra high speed optical communication system, optical phase modulation systems have been attracting much attention. Whereas data modulation is applied to the light intensity of transmission laser light in conventionally-used optical intensity modulation methods, data modulation is applied to the phase of transmission laser light in optical phase modulation systems. QPSK (Quadrature Phase Shift Keying) method, 8PSK (8-Phase Shift Keying) method and the like are known as optical phase modulation systems. In the optical phase modulation systems, the symbol rate (baud rate) can be lowered by allocating a plurality of bits to one symbol. As a result, an operation speed of electrical devices can be reduced and thus cutting of device production costs becomes possible. If QPSK scheme is used, for example, two bits (for example, 00, 01, 11, and 10) are allocated to each of four optical phases (for example, 45, 135, 225, and 315 degrees). As a result, the symbol rate in QPSK scheme can be reduced to one-half of the symbol rate in optical intensity modulation methods (that is, bit rate).
FIG. 15 is a diagram called a constellation map of QPSK in which four symbols of QPSK scheme and bit sequences allocated to each symbol are illustrated on a phase plane. The numerical values on the vertical axis and the horizontal axis correspond to values of the magnitude of amplitude in signal waveform multiplied by 10×21/2. Correlating a bit sequence to each symbol in the optical phase modulation system is called symbol mapping. Although the case will be described below where QPSK scheme is used as an optical phase modulation system, other optical phase modulation systems are also applicable.
To receive the signal light modulated with optical phase modulation, an optical coherent system is used. That is to say, laser light with almost the same optical frequency as that of signal light (which is called local light) and the signal light are coupled by an optical element called a 90-degree hybrid, and the output is received by an optical detector. In the following description, it is assumed for simplicity that each polarization state of the signal light and the local light is an identical linear polarization.
If the optical coherent system is used, an alternating current component of an electrical signal outputted from the optical detector is a beat signal composed of the signal light and the local light. The amplitude of the beat signal is proportional to light intensities of the signal light and the local light, and the phase of that is equal to the difference in the optical phase between the signal light and the local light provided that a carrier wave frequency of the signal light and an optical frequency of the local light are the same. At this time, if the optical phase of the local light is the same as that of laser light inputted into the optical modulator of an optical transmitter, the phase of the beat signal becomes equal to the optical phase applied to the laser light in the optical transmitter. Accordingly, it is possible that transmitted data are demodulated by transforming the phase of the beat signal into a bit sequence using symbol mapping. That is to say, if an optical signal with the constellation in FIG. 15 is transmitted from the optical transmitter, an optical receiver is able to receive a signal with the similar constellation.
However, in general, the value of the carrier wave frequency of the signal light does not completely coincide with that of the optical frequency of the local light. Moreover, the optical phase of the local light in the receiver does also not coincide with that of the laser light inputted into the optical modulator in the optical transmitter. Here, the optical phase difference between the laser light inputted into the optical modulator in the optical transmitter and the local light is called an optical phase excursion, and the difference between the carrier wave frequency of the signal light and the optical frequency of the local light is called an optical carrier wave frequency excursion.
FIGS. 16A and 16B show constellation maps in cases that there exist the optical phase excursion and the optical carrier wave frequency excursion. In these figures, the numerical values on the vertical axis and the horizontal axis correspond to values of the magnitude of amplitude in signal waveform multiplied by 10×21/2. As shown in FIG. 16A, if there exists an optical phase excursion, a signal is received which has a constellation rotated by the amount of the optical phase excursion compared to the constellation shown in FIG. 15. Since it is impossible to know the value of an optical phase excursion in advance, a problem arises that incorrect data are demodulated if a symbol is transformed into a bit sequence using the symbol mapping shown in FIG. 15 as it is.
If there is an optical carrier wave frequency excursion further, the phase of the above-described beat signal becomes equal to the value which is obtained by adding the optical phase excursion to the product of the optical carrier wave frequency excursion and the receipt time. As a result, as shown in FIG. 16B, a signal is received which has a constellation with the constellation shown in FIG. 15 rotating temporally. In this situation, since the phase of the beat signal varies temporally, it is impossible to demodulate the data out of the phase of the beat signal using the symbol mapping shown in FIG. 15. Therefore, in an optical phase modulation systems, a function for compensating a phase excursion and a carrier wave frequency excursion is required which prevents a constellation from rotating owing to an optical phase excursion and an optical carrier wave frequency excursion (refer to, for example, patent literature 1).    Patent Literature 1: Japanese Patent Application Laid-Open Publication No. 2008-271527 (paragraphs [0012]-[0030])